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PostgreSQL – ALTER TABLE

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In PostgreSQL, the structure of an existing table can be modified using the ALTER TABLE statement.
Syntax: ALTER TABLE table_name action;
PostgreSQL supports the various actions to perform with ALTER TABLE as listed below:
  • Add a column to an existing table as below:
    ALTER TABLE table_name ADD COLUMN new_column_name TYPE;
  • Drop a column from an existing table as below:
    ALTER TABLE table_name DROP COLUMN column_name;
  • Rename a column from an existing table as below:
    ALTER TABLE table_name RENAME COLUMN column_name TO new_column_name;
  • Change the default value of a column from an existing table as below:
    ALTER TABLE table_name ALTER COLUMN column_name [SET DEFAULT value | DROP DEFAULT];
  • Change NOT NULL constraint as below:
    ALTER TABLE table_name ALTER COLUMN column_name [SET NOT NULL| DROP NOT NULL];
  • Add CHECK constraints to a column as below:
    ALTER TABLE table_name ADD CHECK expression;
  • Add a constraint as below:
    ALTER TABLE table_name ADD CONSTRAINT constraint_name constraint_definition;
  • Rename an existing table as below:
    ALTER TABLE table_name RENAME TO new_table_name;
Now let’s look into an example. For the same purpose let’s first create a table (say, links) that we can alter in our example using the below statement:
CREATE TABLE links (
    link_id serial PRIMARY KEY,
    title VARCHAR (512) NOT NULL,
    url VARCHAR (1024) NOT NULL UNIQUE
);
Example: In this example, we will insert a new column to the links table as below:
ALTER TABLE links
ADD COLUMN active boolean;
The following statement removes the active column from the links table:
ALTER TABLE links 
DROP COLUMN active;
To rename the title column to link_title, you use the following statement:
ALTER TABLE links 
RENAME COLUMN title TO link_title;
The following statement adds a new column named target to the links table:
ALTER TABLE links 
ADD COLUMN target VARCHAR(10);
To set _blank as the default value for the target column in the links table, you use the following statement:
ALTER TABLE links 
ALTER COLUMN target
SET DEFAULT '_blank';
If you insert the new row into the links table without specifying a value for the target column, the target column will take _blank as the default value. For demonstration let’s insert some data as follows:
INSERT INTO links (link_title, url)
VALUES('PostgreSQL Tutorial', 'https://www.geeksforgeeks.org/');
Query data from the links table:
SELECT * FROM links;
Output:

Last Updated : 28 Aug, 2020
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